This invention relates to medical ultrasonic imaging systems and, in particular, to capacitive micromachined ultrasonic transducers for such systems.
The ultrasonic transducers used for medical imaging have numerous characteristics which lead to the production of high quality diagnostic images. Among these are broad bandwidth and high sensitivity to low level acoustic signals at ultrasonic frequencies. Conventionally the piezoelectric materials which possess these characteristics and thus have been used for ultrasonic transducers have been made of PZT and PVDF materials, with PZT being the most preferred. However PZT transducers require ceramic manufacturing processes which are uniquely different from the processing technologies used to manufacture the rest of an ultrasound system, which are software and semiconductor intensive. It would be desirable from a manufacturing standpoint to be able to manufacture ultrasonic transducers by the same processes used to fabricate the other parts of an ultrasound system.
Recent developments have led to the prospect that medical ultrasound transducers can be manufactured by semiconductor processes. These developments have produced capacitive micromachined ultrasonic transducers or cMUTs. These transducers are tiny diaphragm-like devices with electrodes that convert the sound vibration of a received ultrasound signal into a modulated capacitance. For transmission the capacitive charge is modulated to vibrate the diaphragm of the device and thereby transmit a sound wave. Since these devices are manufactured by semiconductor processes the devices have dimensions in the 10-200 micron range. However, many such devices can be grouped together and operated in unison as a single transducer element.
Since cMUTs are very small, it is desirable that constructed cMUTs have as great a response to received acoustic signals as possible. A cMUT should desirably exhibit as large a capacitive variation as possible to received signals. One approach to increasing the capacitive variation is to use electrodes only at the center of the cMUT diaphragm which will cause the capacitive charge to be located only at the center of the moving diaphragm. However, this arrangement requires the use of very small conductive paths to the electrodes, which increases the impedance of these paths and thereby limits the response of the cMUT. It is desirable to be able to increase the capacitive variation of a cMUT without the use of such high impedance conductive paths.
One of the advantages of cMUT transducers is that they can be made using semiconductor fabrication processes. Accordingly, cMUTs have been fabricated using silicon and glass substrates for the base of the transducers. These substrates form the back of the transducers opposite the transmitting surface. Since transducers are intended to transmit most of their energy out from the transmitting surface without radiating appreciable acoustic energy out the back of the transducers or into neighboring transducers through lateral coupling, a backing layer is usually applied to a transducer to damp or attenuate this undesired acoustic energy. Accordingly it would be desirable to be able to fabricate cMUTs using materials which are better suited to reducing or eliminating this unwanted energy coupling.
cMUTs have been found to exhibit a response to applied transmit signals which is nonlinear due to the nonlinear electromechanical response of the charged cMUT diaphragm, which causes a corresponding quadratic signal variation. Such a nonlinear response will result in distortion in the transmit signal. This distortion can manifest itself as signal components in the harmonic band of the desired transmit pulse, which can appear in the received echo signal as unwanted interference. It is desirable to prevent such distortion from contaminating received echo signals.
A cMUT transducer is conventionally operated with a bias voltage which causes the transducer to have a range of operation which is not quadratic. The bias voltage must be carefully controlled so as to maintain high transducer sensitivity without short-circuiting the transducer""s capacitance. It is desirable to be able to maintain the applied bias in a condition which is stable in the presence of long-term effects that can cause transducer short-circuits.
In accordance with the principles of the present invention a cMUT transducer is described with improved signal response. The improved response arises by reason of a nonplanar floor of the cMUT cell, which concentrates the cellular charge in the vicinity of that portion of the cell diaphragm which is most responsive to applied and received signals. A manufacturing process for cMUT transducers is described which enables the transducer to be fabricated by a technique of micro-stereolithography using polymeric materials. In operation the cMUT is biased by a controlled bias charge rather than a bias voltage. The transmission of unwanted signal components in the harmonic band is minimized by the use of predistorted transmit signals that counteract the transducer""s nonlinear response.